1. Field of the Invention
The present invention relates to power distribution, more specifically, to power regulation and conditioning.
2. Background Art
In the current power environment, power plants are interconnected to loads via utility grids to deliver large amounts of power. Efficient distribution over long distances power is delivered as low frequency three phase AC current. However, low frequency AC current is not suitable for end use at some loads. Therefore, prior to the end use, the utility grid power has to be converted to a more useable form. A typical power xe2x80x9cconditioningxe2x80x9d configuration includes an AC-to-DC rectifier that converts the utility AC power to DC across positive and negative DC buses, across a DC link, and an inverter linked to the DC link that converts the DC power back to three phase AC power having an end useable form, namely three phase high frequency quality AC voltage. A controller controls the inverter to provide the voltage waveforms required by the load.
Besides power plants connected to a grid, local power systems can be interconnected to the grid. And, backup power supplies provide power to the local load when utility grid power is cut off. In the distributed grid, the local power system transfers power to the grid or takes power from the grid as needed. There are many forms of local power systems such as fuel cells and microturbines, each having particular requirements and operating parameters. Thus, there are many applications for an efficient power conditioning unit that can generate the proper power to the load with minimal distortion.
Typical DC-DC voltage regulators are well-known in the art, and they primarily provide a constant DC voltage to an inverter which then pulse width modulates (PWM""s) the DC voltage to produce an AC output voltage. PWM inverters and various techniques that convert DC to AC are also well known in the art. A PWM pattern is generally a set of switching transients that is applied to the DC signal via an inverter and produces a sinusoidal AC signal.
The combination of an inverter stage connecting to the DC-DC converter is also commonplace, and the inverter converts the cleaned DC output of the converter into an AC output of the inverter.
As recognized in the industry, there are many limitations in the existing designs and considerable room for improvement. For example, the systems have difficulty addressing unbalanced load conditions, there are problems with ripple frequencies that require expensive and large inductors and capacitors. There have been various attempts in the prior art to regulate the AC output signal and reduce distortions and ripple on the AC output. Typically these attempts measured the voltage and current of the AC signal and changed the PWM switching patterns to minimize distortions.
In a typical application, the DC-DC converter conditions and regulates power from an regulated DC source and tries to produce a tightly regulated DC output voltage for use by a PWM output inverter. In a 240/120VAC 60 Hz application, the output power (or DC current) as seen by the dc link occurs at a 120 Hz frequency. Typically an output PWM filter is used to filter off the PWM inverter xe2x80x9cswitching ripplexe2x80x9d thereby producing a xe2x80x9ccleanxe2x80x9d sinusoidal output. Voltage and consequently power loss developed across the PWM filter inductor increases as a function of the difference between the DC link volts and the output AC voltage. The greater the difference between the DC links voltage and the output voltage, the more extreme the voltage levels across the output filter components.
U.S. Pat. Nos. 4,935,859 and 4,935,860 describe a VCSF system that has an inverter that applies a PWM pattern of switching transients to the DC signal to produce an AC signal. A feedback circuit reduces short duration switching transients in the PWM pattern by analytically determining DC link distortion. These patents relate to methods for reducing insulated gate bipolar transistor (IGBT) short pulses by modulating the DC link. A related patent, U.S. Pat. No. 4,937,720, is for a DC link harmonic elimination for AC inverters.
There are other inventions intended to reduce semiconductor switching losses with a variety of soft switch techniques, such as U.S. Pat. Nos. 5,559,685, 5,592,371 and 5,841,644. While these various schemes have certain advantages, they are not intended nor allow for reducing filter inductor losses.
Besides utility and local power conditioning, there are many other applications for power conditioning such as uninterruptible power supplies (UPS), that benefit from more efficient conditioning.
What is needed is a system that can reduce the voltage or switching ripple across the PWM output filter inductor. For safety purposes, the system should also provide fast circuit breaking in fault conditions. Such a system should also allow for lower cost, lighter weight and more efficient PCU systems.
The present invention has been made in consideration of the aforementioned background. One object of the invention is a modulated dc link scheme for dramatically reducing the switching frequency ripple developed across the PWM output filter inductor.
Another object of the invention is to allow greater overall power conditioning unit (PCU) inverter efficiency because of the reduced PWM filter inductor losses, without any loss of AC output waveform dynamic performance. Typically a voltage xe2x80x9cfeed forwardxe2x80x9d term from the dc volts feedback to the PWM controlling DSP can allow for stable inverter control.
A further benefit is that lower cost PWM filter inductor core materials may be used as the core losses are primarily driven by the high frequency ripple content of the PWM output. Inductor size and weight may also be reduced.
During excessive overloads such as occur during a circuit breaker protected branch short circuit where, the power conditioning inverter is required to feed energy into the fault to cause the protection circuit breaker to trip. Utilizing the dc modulation scheme of the present invention automatically reduces the DC link voltage to track the output AC volts. When the load impedance is very small, or shorted, the output AC volts is consequently reduced to limit output current. The output inverter provides more energy faster into the fault to clear the circuit breaker. This is primarily because the inverter switch losses are reduced (reduced VDC), and the filter inductor magnetic core normally absorbs a large percentage of the total output power during an overload fault.
Further passive component cost reductions, and lifetime enhancements can be realized with this dc link modulation scheme. For example DC link bulk capacitance can be reduced as the required energy storage is reduced, thereby allowing for smaller and less expensive capacitors. PWM filter costs (capacitors and inductors) are also reduced. And, the longevity and stability of the system is enhanced by avoiding excessive power handling primarily due to undesirable switching ripple.
Electromagnetic interference (EMI) is also reduced as the PWM output voltage ripple, and therefore higher frequency harmonics (up into the RF range), are dramatically reduced. This allows the output EMI/RF filter to be more efficiently designed, while achieving similar EMI performance. Conversely, a high quality RE/EMI filter may be retained thereby providing significant EMI/RF emissions reductions. In certain applications that require low noise emissions, such as military applications, the present scheme has particular applicability.
The dc modulation scheme allows very efficient power conditioning at a lower cost, particularly when using of a high frequency transformer (20-50 kHz) in the main DC/DC converter. High-frequency links have rapidly become the preferred technology in grid-connected, photovoltaic inverter applications, and have the advantage of providing dc isolation and inversion without the need of 60 Hz transformer. As a result, the designs are smaller and lighter, and advantages are further described in U.S. Pat. No. 4,641,232.
In one embodiment of the present invention, the system uses a MOSFET based H-bridge topology. The high frequency transformer, cost, size, weight are dramatically reduced as compared to a 60 Hz transformer. The high frequency transformer provides isolation, and allows for a voltage transform (from 48Vdc to 400 VDC, etc.).
The dc modulation scheme of the present invention can also be used to provide AC-AC transformations even with a 1:1 turns ratio in order to provide isolation only, thereby reducing transformer cost, size etc. This has vast applications in avionics, space, and other applications where size and weight limitations are critical.
The preferred embodiment of the present invention is in DC-DC-AC inverters, or power conditioner units (PCU""s). However, the present invention also allows use in AC-DC-AC inverters, for power conditioning or motor controllers. These inverters can be used in residential, commercial, and even industrial applications. Although most beneficial in single/2phase power generator or motor systems (such as in appliance single phase motors). Additional benefits are also achieved in 3-phase or poly-phase power systems.
Although this dc link modulation scheme benefits 3 phase inverters, the most substantial benefits are realized with single/2 phase three wire power inverters, such as are normally required for residential power generation systems. This is because the single/2 phase (120 Volt L-N, 240 Volt L-L) output power is produced at a 2xc3x97 fundamental frequency rate, or 120 Hz, wherein the fundamental frequency is 60 Hz.
In a normal 3-phase system, each phase voltage is 120 degrees phase shifted, whereas the residential voltages are 180 degrees phase shifted. Therefore, in residential power generators/inverters the DC link current can be seen to contain a large magnitude of 120 Hz.
There are several basic embodiments to tooth modulator equipped three phase inverters. In a first embodiment, each phase is equipped with a separate DC supply (tooth modulator). In this case, each individual phase PWM inverter may consist of a single xc2xd bridge, or preferably an H-bridge.
A second embodiment is where all three PWM phases are fed from a common DC supply consisting of two series connected tooth modulators. In this case, each tooth modulators controls xc2xd of the DC link voltage supplied to the inverter switches. The upper DC supply, or top, tooth modulator will track the sum of the positive xc2xd of the three phase voltages line to neutral. The lower DC supply, or bottom, tooth modulator will track the sum of the negative xc2xd of the three phase voltages line to neutral. Each tooth modulator thereby supplies a DC with either 300 Hz, or 360 Hz content.
Switched voltages applied across the output PWM inductor increase as a function of the difference between the applied DC link volts and the output AC voltage. Consequently power losses developed in the PWM inductor peak near the zero crossing of the output AC sinusoidal voltage. The tooth modulator derived DC supply tracks the AC output voltage, reducing the difference between the applied DC volts and the AC output volts, thereby reducing inductor core losses.
It is also within the scope of the invention and contemplated by the inventors to use the modulated DC link volts to feed a thyristor type inverter, as opposed to an IGBT based output inverter. This allows further cost reduction while not compromising too much on the power quality. In another embodiment, three phase inverters (commonly fed from a single dc link) are fed from different, independently controlled dc links that allow use of IGBT, or even thyristor based output inverters. This has enormous benefits for higher powered fuel cell fed power conditioning inverter systems.
Without a load present, the dc modulation scheme regulates the output volts to a fixed dc volts, and under no-load conditions there would be virtually no PWM filter losses. This approach is consistent with a xe2x80x9csleep-modexe2x80x9d where the PCU output, under no-load conditions, allows the AC output to dwell at a low DC voltage (xcx9c5-10% of nominal AC rating). When a load is applied, the PCU detects the resulting current output (or the collapse of the applied DC volts) and then goes on line and begins to produce the nominal regulated AC output voltages. In certain application some limited output load may be required.
Some phase shift between the VAC out feedback and the response of the H-bridge (DC-DC converter) may occur, but this is corrected with appropriate control loop design commonly known in the art, for example, adding phase lead network to the feedback amplifier.
There is much interest industry wide in efficiency enhancement technologies for use in power inverters. Those interested in this technology include fuel cell power systems developers (mostly residential), power supply manufacturers, diesel and microturbine generators, makers of Uninterruptible Power Supplies (UPS), and essentially any application that requires electrical conversion.
One object of the invention is a dc links tooth modulator for producing an AC output, comprising a converter producing a converter output signal, wherein the converter has an input end and an output end, wherein the input end is connected to a power source. A control feedback circuit is used for delivering a composite voltage error signal to a pulse width modulator controller. There is an output inverter connected to the output end of the converter and communicating to the pulse width modulator controller for pulse width modulating the converter output signal and producing an inverter output signal at a fundamental frequency with a dc links signal at a multiple of the fundamental frequency, wherein the dc links signal tracks an envelope of the inverter output and is offset from the inverter output by a dc term.
Additionally, an object includes the dc links tooth modulator, further comprising an offset generator producing the dc term.
An object includes comprising an EMI/RF filter section connecting to the inverter output signal.
Another object is the dc links tooth modulator, wherein the inverter is a DC-AC inverter.
Yet another object is the dc links tooth modulator, wherein the system is under a no-load condition and the output signal remains at a low fixed level until a load is re-established.
An object includes wherein the output inverter is selected from the group consisting of thyristor type inverters and IGBT based output inverters.
One object is a dc links tooth modulation topology for a poly-phase AC output, comprising a converter producing a converter output signal, wherein the converter has an input end and an output end, wherein the input end is connected to a power source, and wherein a voltage sensor measures a voltage feedback signal. An inverter is connected to the output end of the converter with a pulse width modulator controller for pulse width modulating the converter output signal and producing an inverter output. There is a control feedback circuit delivering a composite voltage error signal to the pulse width modulator controller, wherein the composite voltage error signal comprises a first feedback signal proportional to the inverter output summed with the voltage feedback signal, and wherein the inverter output is an AC signal at a fundamental frequency with a dc links signal at a multiple of the fundamental frequency offset by a dc term. Also, further comprising a switch, wherein the switch connects to a reference generator to produce the inverter output signal. And also including an offset generator, wherein the offset generator produces the dc term.
An object includes the dc links tooth modulation topology, wherein the feedback signal proportional to the inverter output is produced by the process of taking an absolute value of the inverter output signal to produce a positive output signal, inverting the positive output signal to produce an inverted output signal, and summing the inverted output signal with a dc level to produce a tooth modulated signal.
In addition, the dc links tooth modulation topology, further comprising an imbalance limiter summed to the voltage feedback signal.
An object of the invention is a power conditioning system, comprising a power source, a dc-dc converter producing a dc output signal, wherein the dc-dc converter has an input end and an output end, and wherein the input end is connected to the power source. An output inverter is connected to the output end of the dc-dc converter for pulse width modulating the dc output signal and producing an inverter output voltage at a fundamental frequency. There is a control loop feedback signal connecting to the output inverter, wherein the control loop feedback signal is proportional to the inverter output voltage, and wherein a dc link voltage is a multiple of the fundamental frequency.
In addition, the power conditioning system, wherein the dc-dc converter comprises a dc capacitor connected parallel to the power source, a plurality of power switches connected parallel to the dc capacitor, a high frequency step up transformer connected to the plurality on a first side, a rectifier section connected to a second side of the transformer, and a filter section connected to the rectifier section.
There are currently no similar commercial devices in the marketplace that employ any form of DC links modulation scheme as described herein, although there are some power factor (PF) correction type control schemes, but these are different in purpose and function. Such devices do not pertain to the DC link voltage modulation scheme described herein.
Other objects, features and advantages are apparent from description in conjunction with the accompanying drawings.